1. Field of the Invention
The present invention relates to a transmitter apparatus that modulates a carrier wave to produce a modulated wave having communication data superimposed thereon and that then transmits the modulated wave to a receiver apparatus. The present invention relates also to a communication system employing such a transmitter apparatus.
2. Description of the Prior Art
In recent years, IC cards having a semiconductor integrated circuit device mounted on a card base have been becoming increasingly popular. IC cards can perform data communication between the semiconductor integrated circuit device mounted thereon and an external reader/writer apparatus, and can store necessary data in a non-volatile memory incorporated in the semiconductor integrated circuit device mounted thereon.
In particular, multiple-purpose IC cards provided with a large-capacity memory so as to be able to store a plurality of application programs can serve multiple purposes as cannot be served by conventional magnetic cards or the like, and therefore such IC cards are adopted in information communication systems in a variety of fields (such as the finance, distribution, transportation, and medical industries).
One main advantage of a non-contact-type IC card system as described above is that there is no need to provide contact terminals in reader/writer apparatuses or IC cards. This reduces the occurrence of faults, such as breakage of contacts, and thus helps reduce maintenance costs and enhance usability.
Another main advantage of a non-contact-type IC card system is that it permits the building of a transaction processing system that is easier and quicker to operate than one based on a contact-type system. For example, when a non-contact-type IC card system is adopted in a ticket examination system for a railway or bus network, it is possible to examine a ticket easily and quickly by asking a passenger to hold the ticket (an IC card) over or around a ticket examination gate (a reader/writer apparatus) (this operation will hereinafter be referred to as “holding-over” operation) or put the ticket into momentary contact with the ticket examination gate (this operation will hereinafter be referred to as “touch-and-go” operation).
Incidentally, non-contact-type IC card systems are classified, according to the distance over which communication takes place, into close, near, and other types. For each of these types, the format to be used in data communication is currently in the process of being standardized under ISO/IEC14443 and ISO/IEC15693.
FIGS. 4A and 4B are waveform diagrams illustrating the data communication format complying with the ISO/IEC14443 standard. FIG. 4A shows a demodulated waveform observed in an IC card or reader/writer apparatus, and FIG. 4B shows an outline of character data communicated.
As shown in FIG. 4A, the data communication format complying with the ISO/IEC14443 standard is designed to permit frame-by-frame communication of information, in which each frame consists of an SOF [start of frame] indicating the head of the frame, followed by character data complying with a predetermined character transmission format, followed by an EOF [end of frame] indicating the end of the frame. Here, as shown in FIG. 4B, the character transmission format is designed to permit continuous communication of data corresponding to as many sets of one-byte data as the number of characters to be communicated, with each byte having a start bit and a stop bit added thereto. In this data communication format, when no communication is taking place, the communication data is kept in a predetermined logic state (hereinafter referred to as a mark state), and meanwhile the receiver apparatus remains in a state waiting for data communication.
It is true that, in the data communication format described above, the receiver apparatus can recognize the start of data communication by detecting a transition of the received data from a mark state (a logic “H”) to a fall due to an SOF (a logic “L”).
However, in the data communication format described above, if the communication data is not kept in a mark state (at a logic “H”) when no communication is taking place, it is not possible to detect an SOF. This leads to failure in data communication.
FIGS. 5A to 5E are waveform diagrams illustrating erroneous detection of an SOF. In a transmitter apparatus complying with the ISO/IEC14443 Type B standard, a carrier wave (having a carrier frequency of 13.56 MHz) is subjected to 10% amplitude modulation (hereinafter referred to as 10% ASK (amplitude shift keying)) to produce a modulated wave (a′) having communication data superimposed thereon, and this modulated wave (a′) is transmitted to the receiver apparatus. It is to be noted that, in 10% ASK, when a carrier wave is modulated, its amplitude is reduced to about 90% of its original amplitude in the unmodulated state.
In the receiver apparatus, the modulated wave (a′) is subjected to full-wave rectification to obtain a rectified wave (b′), which is then integrated to produce an envelope wave (c′). The envelope wave (c′) is then differentiated to obtain a differentiated wave (d′), which is then compared with threshold levels Vth1 and Vth2 (VDD/2±ΔV) to produce a binary demodulated wave (e′) representing the communication data. That is, in the receiver apparatus, slight fluctuations in the rectified wave (b′) are detected to produce the demodulated wave (e′).
Here, if, as indicated by broken lines in these figures, the demodulated wave (e′) is kept in a mark state (at a logic “H”) when no communication is taking place, the receiver apparatus can recognize the start of data communication by detecting a transition of the demodulated wave (e′) from a mark state (a logic “H”) to the fall due to an SOF (a logic “L”).
On the other hand, if, as indicated by solid lines in the figures, ringing noise or the like is superimposed on the modulated wave (a′) when no communication is taking place, a distortion appears in the rectified wave (b′) and in the envelope wave (c′). This causes an unintended noise pulse to appear in the differentiated wave (d′).
Even then, so long as the lower peak of the noise pulse does not reach below the threshold level Vth2 or its upper peak reaches above the threshold level Vth1, once the noise pulse disappears, the demodulated wave (e′) is kept back in a mark state (at a logic “H”). Thus, no erroneous detection of an SOF results.
However, if, as shown in the figures, the lower peak of the noise pulse reaches below the threshold level Vth2 and in addition its upper peak does not reach above the threshold level Vth1, an unintended transition from a mark state (a logic “H”) to a logic “L” occurs in the demodulated wave (e′), which is thereafter kept in the latter logic state even after the disappearance of the noise pulse. With the demodulated wave (e′) kept at a logic “L” in this way when no communication is taking place, it is impossible to detect the falling edge of an SOF. This results in failure in communication.
In particular, in low-depth modulation (modulation in which the ratio of the maximum amplitude to the minimum amplitude of the modulated wave (a′) is comparatively low) such as 10% ASK, the slightest noise can be erroneously recognized as data. This makes the above-described failure more likely.
Moreover, since non-contact-type IC cards are supplied with electric power by way of a radio wave, their modulation/demodulation operation tends to be unstable because of fluctuations in the electric power they receive. This makes quite likely failure similar to that described above.